PLD技术制备ZnO薄膜及其结构和发光性质研究

【作者】 孙柏；

【导师】 徐彭寿；

【作者基本信息】 中国科学技术大学 ， 核技术及应用， 2007， 博士

【摘要】 ZnO是一种纤锌矿结构的宽禁带半导体材料，室温下的禁带宽度约为3.37eV，具有优异的光学和电学特性，在透明导电薄膜，表面声波器件、气体传感器和光电器件等方面有着广泛的应用，尤其是高质量ZnO薄膜的室温紫外受激发射的实现，使其成为当前的研究热点。本论文利用PLD技术在Si，SiC，Al₂O₃衬底上制备了ZnO薄膜，并对工艺进行优化，利用一些常规测试方法和同步辐射实验技术研究了不同生长条件对PLD技术制备的ZnO薄膜结构，光学和电学性质的影响。主要的研究工作及结果如下：1．在Si衬底上ZnO薄膜的生长及其结构和发光性质的研究利用PLD方法，在Si衬底上制备出了单一取向的ZnO薄膜，研究了衬底温度，氧气氛，激光能量和脉冲频率等生长条件对ZnO薄膜的结晶质量的影响，分析了这些影响产生的原因，并利用优化的生长条件制备出了高质量的ZnO薄膜，其（002）峰双晶摇摆曲线的半高宽为1.3°。利用同步辐射XAFS和XPS研究了两个不同衬底温度下（300℃，500℃）生长的ZnO薄膜的局域结构以及薄膜表面元素的化学态和相对含量。研究结果表明，500℃生长的薄膜的结晶质量要好于300℃生长的样品，但500℃生长的薄膜中的O／Zn比却小于300℃生长的薄膜。利用GID研究了薄膜内部不同深度的晶格驰豫过程，结果显示随着X射线探测深度从靠近薄膜表面增加到薄膜与衬底的界面处，两个样品a方向的晶格常数都减小，这说明薄膜内部的应变是不均匀的。PL谱结果表明，ZnO薄膜的紫外发射与它的晶体质量有着非常密切的关系。在用同步辐射作激发源的低温下的光致发光谱中，发现了发光中心位于430nm的紫光发射，我们认为该发射与存在于ZnO—ZnO晶粒间界的界面势阱所引起的界面缺陷能级到价带的跃迁有关，这个界面势阱可能起源于Zn填隙。2．Mn掺杂对ZnO薄膜结构和光学性质的影响利用PLD方法在Si（111）衬底上生长出了Mn掺杂的c轴高度取向的ZnO薄膜。光致发光（PL）结果显示了Mn的掺杂引起了薄膜的带边发射蓝移，强度减弱，紫光发射几乎消失，但绿光发射增强。利用XRD，XAFS，XPS和Raman等实验技术对Mn掺杂的ZnO薄膜的结构进行了研究。XRD和XAFS结果表明Mn进入了ZnO的晶格，处在Zn²⁺的替代位置形成了Zn_0.9Mn_0.1O合金薄膜，XAFS和XPS结果从实验上证实了Mn是以+2价的价态存在的，这就导致了掺Mn以后的薄膜带隙变大，在发光谱中表现为带边发射的蓝移。Raman结果表明Mn的掺入对薄膜的晶格振动产生了一定的影响，Zn_0.9Mn_0.1O合金薄膜与衬底之间的应力要比未掺杂的ZnO薄膜的大。由于掺入的Mn⁴⁺与薄膜中的填隙Zn反应自身变为Mn²⁺，导致薄膜的结晶性变差，薄膜中的填隙Zn减少，O空位增多，引起带边发射和紫光发射减弱，绿光发射增强。3．在SiC或以SiC为缓冲层的Si上生长ZnO薄膜及ZnO／SiC界面研究采用以MBE方法在Si衬底上生长的3C—SiC作为过渡层，利用PLD方法制备了高度c轴取向的ZnO薄膜，研究了衬底温度，氧分压对ZnO薄膜的结晶质量的影响，并分析了产生这些影响的原因。通过对PLD方法的工艺优化，在6H-SiC单晶衬底上制备出了高质量的ZnO薄膜，X射线双晶摇摆曲线结果显示其（002）衍射峰的半高宽仅为0.47°。同步辐射掠入射X射线衍射结果表明该ZnO薄膜和衬底之间的平行于衬底表面a轴方向的实际的晶格失配度仅为5.84％，并且利用X射线Φ扫描技术观察到了该ZnO薄膜（110）等效晶面的六重对称性。由此说明，我们已成功地在6H—SiC单晶衬底上制备出单晶ZnO薄膜。此外，我们还以Si衬底上原位生长的SiC和石墨作为过渡层，利用PLD方法制备ZnO薄膜。通过其电学特性的研究表明，SiC和石墨过渡层的使用，可以极大的提高ZnO和Si衬底组成的p—n结的Ⅰ—Ⅴ特性。利用SRPES和XPS的价带谱和芯能级谱技术，研究了金属Zn在SiC表面的吸附和热氧化过程以及ZnO／SiC异质界面的形成和结构。研究结果表明，在SiC表面沉积金属Zn的初始阶段，Zn可以与SiC衬底表面残留的氧结合。随着Zn覆盖度的增加，表面具有金属特性。在氧气氛中，氧可能会以分子的形式吸附在Zn表面，但也可能与Zn成键或化合。在氧气氛中180℃的温度退火后，一部分Zn被氧化形成ZnO，还有少量Zn因受到热蒸发而逸出表面。在氧气氛中600℃温度退火后，形成的ZnO会阻止金属Zn逸出表面，覆盖的金属Zn全部被氧化而生成ZnO。而在氧气氛中退火时，衬底也会发生轻度氧化，从而导致在ZnO／SiC界面处存在一层很薄的Si的氧化层。根据得到的光电子能谱的实验结果，计算出用Zn热氧化方法形成的ZnO／SiC异质结的价带偏移为1.1eV。4．在Al₂O₃上生长ZnO薄膜及其界面结构的研究在Al₂O₃衬底上，利用PLD方法在不同的衬底温度和不同的氧分压下生长了高度c轴取向的ZnO薄膜。XRD和RHEED结果显示了衬底温度和氧分压对ZnO薄膜的生长模式和结晶质量有很大的影响。X射线双晶摇摆曲线结果显示利用PLD方法以Al₂O₃为衬底，在优化的条件下制备的ZnO薄膜的（002）衍射峰的半高宽仅为0.46°，已经达到了单晶水平。同步辐射掠入射X射线衍射结果显示，随着X射线穿透深度从薄膜表面增大到薄膜和衬底的界面处，450℃和650℃生长的ZnO薄膜的a方向的晶格常数明显增加，而750℃的ZnO薄膜的a方向的晶格常数却略微减小，这说明衬底温度对ZnO薄膜内部不同深度的晶格驰豫有很大的影响。同步辐射掠入射X射线反射结果显示650℃，0.13Pa条件下生长的ZnO薄膜的厚度为46.2nm，表面粗糙度为0.63nm，薄膜与衬底界面的粗糙度为1.41nm。更多还原

【Abstract】 ZnO is a wide band gap semiconductor of wurtzite structure with a band gap of 3.37eV at room temperature , it has many applications, such as transparent conducting films, surface acoustic wave devices, gas sensors and photoelectronic devices. The report of ultraviolet laser emission of high quality ZnO thin films at room temperature promoted people’s interesting in ZnO researching.In this thesis, we report that ZnO thin films on different substrates of Si, SiC and Al₂O₃ are prepared by pulsed laser deposition and the growth conditions are optimized. Furthermore we employed some normal analysis methods and synchrotron radiation experiment techniques to investigate the influence of different growth conditions on structure, optical and electric properties of ZnO thin films. The major results are listed as the following:1. Growth of ZnO thin films on Si substrate and investigation of their structure and optical propertiesWe have prepared highly c-axis oriented ZnO on Si substrate at different growth conditions, such as substrate temperature, oxygen partial pressure, laser energy density and laser frequency, and investigated the effects of the growth condition on the crystallinity of ZnO thin films, and analyzed the reason of the influence. Then we prepared ZnO thin film with good crystallinity at the optimized grow condition. X-ray ω rocking curve result indicates that the FWHM of its （002） peak is 1.3° .Moreover we used XAFS and XPS to study the local structure, chemical state and element relative content of ZnO grown at different substrate temperature （300°C and 500°C）.The results of XAFS and XPS show that the crystallinity of the ZnO thin films grown at 500°C is better than that of the ZnO thin films grown at 300℃,but the O/Zn proportion of the ZnO thin films grown at 500°C is smaller than that of ZnO thin films grown at 300℃.GID was used to investigate the lattice relaxation inside thin films, its result indicated that both a-axis lattice constants of the two sample decreased with the increasing of the detected depth of X-ray from surface of thin films to the interface between films and substrate, which shows the stress inside the thin films is not equal . Furthermore, PL results suggest that UV emission of ZnO is relative to its crystallinity. We also observed the violet emission with about 430nm of ZnO films excited by synchrotron radiation at 18K. The violet luminescence is ascribed to the defects related to the interface traps existing at the grain boundaries, which probably originate from interstitialzinc（Zn_i）.2. Influence of Mn-doped on the structure and optical properties of ZnO thin filmsHighly c-axis oriented Zn_0.9Mn_0.1O thin films have been fabricated on Si substrates by pulsed laser deposition. Photoluminescence results show that Mn atoms doping induce the blue-shift of UV emission. At the same time the intensity of UV emission decreases, while green emission increases. X-ray diffraction, X-ray absorption fine structure, X-ray photoelectron Spectroscopy and Raman have been employed to characterize the influence of Mn-doping on properties of ZnO thin films. The results indicate that Zn_0.9Mn_0.1O alloy formed after Mn doped in ZnO. Mn atoms have entered into the ZnO crystal lattice and substituted Zn atoms with Mn²⁺ state. As a result, the band gap of Zn_0.9Mn_0.1O increases, which is associated with the UV emission blue-shift. Raman results show that Mn doping influences the crystal lattice vibration of ZnO, and prompts the increasing of stress between film and substrate. Furthermore, the reaction between the doped Mn⁴⁺ and interstitial Zinc atoms （Zn_i） induces the crystallinity of sample worse, and makes Mn⁴⁺ turn to Mn²⁺. Thus, interstitial Zinc atoms （Zn_i） decreases and the oxygen vacancy （Vo） increases, which accounts for the intensity decreasing of UV and violet emission, and the increasing of green emission of the films samples.3. Preparing ZnO thin films on SiC single crystal or SiC buffer and studying the interface of ZnO/SiCWe used PLD technique to prepare highly c-axis oriented ZnO on 3C SiC/Si substrate grown by MBE at different growth conditions, such as substrate temperature, oxygen partial pressure, and investigated the effects of the growth condition on the crystallinity of ZnO thin films. At the same time we also analyzed the cause of the influence. Besides, we have prepared ZnO thin films with good crystallinity on 6H-SiC single crystal substrate at the optimized growth condition by PLD. X-ray ω rocking curve result shows that its FWHM of （002） peak is only 0.47°. The results of synchrotron radiation X-ray grazing incidence diffraction shows the actual lattice mismatch between film and substrate is only 5.84%, and X-ray Φ scan shows that the six-fold symmetry of ZnO（110） planes family, which indicates that we have prepared single crystal ZnO thin film on 6H-SiC single crystal substrate by PLD. We also investigated the effect of SiC and graphite buffer prepared by PLD in situ on the electric properties of ZnO thin films, the result indicates that the buffer can improve the I-V properties of ZnO/Si p-n junction greatly.The adsorption and the thermal oxidation of Zn on 6H-SiC surface and the interface formation of ZnO/SiC have been investigated by using synchrotron radiation photoelectron Spectroscopy （SRPES） and X-ray photoemission （XPS）. The results show that at the initial stage of Zn adsorption, Zn can capture and bond with 0, which was remained on the SiC surface. With increasing of Zn coverage, the surface exhibits metallic characterization. When the deposited Zn film is annealed at 180°C in oxygen flux with the pressure of 2×10^-4Pa, it could be partly oxidized to form ZnO. Part of Zn atoms could be escaped from the surface due to its low evaporation temperature in UHV condition. While annealed at 600°C in same oxygen flux, the total deposited Zn atoms could be oxidized to form ZnO. During annealing process, the substrate is also oxidized, which induces a thin layer of silicon oxide existing at the interface of ZnO/SiC. By using the results of SRPES and XPS, the valence band offset of ZnO/SiC is calculated to be 1.1eV. 4. Preparing ZnO thin films on A1₂O₃ and studying their structure propertiesWe have grown highly c-axis ZnO thin films on Al₂O₃ at different substrate temperatures and oxygen pressures by PLD.XRD and RHEED have been employed to investigate the influence of substrate temperature and oxygen pressure on the crystallinity and growth mode of ZnO thin films. X-ray rocking curve results exhibit that the FWHM of （002） peak of ZnO thin films prepared on Al₂O₃ at the optimized condition is only 0.46°, indicating that we have grown single crystal ZnO thin film. The results of Synchrotron radiation X-ray grazing incidence diffraction show that with the increasing of the detected depth of X-ray from the surface of film to the interface between the film and substrate, the a-axis lattice constants of the ZnO thin films grown at 450°C and 650°C increase obviously, while the a-axis lattice constant of the ZnO thin film grown at 750°C decreases slightly, which indicates that the substrate temperature play important role to affect the lattice relaxation in ZnO thin films. The results of synchrotron radiation X-ray grazing incidence reflection shows that thickness and surface roughness of ZnO thin films grown at 650°C and 0.13Pa are 46.2nm and 0.63nm respectively, and the roughness at the interface is 1.41nm.更多还原